Since renewable energy is achieving more ground lately, one of the issues that appears is storing it.
Fossil fuels themselves are a form of stored energy so the only thing we need to do is extract them. Once we have them, storing it's not a problem.
Green energy on the other hand is different and most of the time it isn't stored. It's delivered right into the grid for immediate consumption.
But if you want to build a true energy independent household (or city) you have to store your renewable energy. And how do you do that?